In certain printing processes, such as silk screen printing, a master screen containing, for example, a negative of a desired image is required. In the printing process this master screen is placed on the surface of the carrier to which the image is to be transferred and ink is imprinted through the master screen.
There are numerous existing techniques for preparing the master screen with one of the most common involving the use of a photographically prepared negative which is placed over a screen onto which has been applied a photo activated emulsion. Such emulsions are typically sensitive to ultraviolet radiation and in this process the screen is exposed to ultraviolet radiation such that the portions of the screen not blocked by the photographic mask are activated. Typically the emulsion is water soluble or at least soluble in a known solvent and in the developing process the non-activated emulsion is removed from the screen thereby leaving a negative of the image. It will be apparent to one skilled in the art that the process can be used to generate a positive of the image.
With ongoing advances in digitized images it is particularly advantageous to directly convert an image from a computer to the master screen. Several methods of performing this conversion have been developed in as much as computer to screen imaging is seen as a method of allowing an operator to modify images or to prepare images based on drawings or other two dimensional formats utilizing a scanning application. Recent improvements in the work flow associated with the actual printing process and the use of digital imaging in the preparation of graphics has made the need for a true CTS an important enabler in order to realise cost benefits produced by other technological improvements.
The prior art methods of preparing master screens using a CTS imaging process include a laser ablation system in which a laser is used to remove material from a fully blocked screen with the non-removed material creating the negative image.
It is also known to use a laser direct imaging in which a laser is scanned point by point over a silk screen coated with a photo activated emulsion to create an image in that emulsion.
As a further known method, ink jet masking is used wherein a negative of the image to be printed is created by using an ink jet to deposit wax onto a screen coated with a photo activated emulsion. The wax blocks the light when the screen is subsequently exposed. Once exposure is completed the wax is removed to produce the final printable image. Another known method is an optical micro electrical mechanical system (MEMS) technique wherein a series of independently controllable mirrors are used to direct light onto a clearly defined and limited area of a screen which has been coated with a photo activated emulsion. Once this area has been activated the mirrors are directed to an adjacent block of the screen and the process repeated. In this manner a full image can be constructed block by block.
U.S. Pat. No. 5,580,698 which issued Dec. 3, 1996 to Anderson describes a system for producing fine printing patterns on large serigraphical printing frames utilising a type of mirror arrangement. In this patent a laser beam is directed through a series of mirrors to a scanner which is moved laterally and longitudinally along sections of a screen and the light source is modulated in order to produce a pattern. The light source is a ultraviolet laser and the pattern is generated in a dot by dot sequence.
In U.S. Pat. No. 6,178,006 a system for plotting a computer stored raster image on a plain photosensitive record carrier is discussed. In this patent the area to be prepared is subdivided into numerous sub areas and each one is processed sequentially.
Each one of the known methods has a number of serious limitations. For example, debris re-deposition is an issue with the laser ablation and like the laser direct image method it is a point by point process. This limits the exposure rate of both methods. With an ink jet approach, wax removal represents another step and another material that must be safely managed and disposed of. Finally, mechanical instability and reliability will be inherent issues with the MEMS method. In fact, this will be true for any projection method.
Accordingly, there is a need for a simple and efficient method of generating a master screen using a computer to screen imaging system.